Install Python versions with previous uv release (#5717)
Part of https://github.com/astral-sh/uv/issues/5713
Shaves 50s or ~25% off the Ubuntu test run. Maybe 30s or 8% off macOS.
Windows already uses the GitHub distributions.
Note this is some of our only test coverage for Python version installs,
we may want to add separate coverage to compensate.
